National Trust director Dame Fiona Reynolds said the proposals focussed on financial gain and could lead to "unchecked and damaging development". She added: "We are deeply concerned that the Government’s proposals allow financial considerations to dominate, which could result in a green-light for poor quality or development in the wrong place.
"We call on the Government to ensure that the economic, environmental and social benefits of development go hand in hand and that in the future it will not become possible to see damaging development pushed through on narrow economic grounds alone."
The Council for Protection of Rural England (CPRE) is also urging the public to oppose the proposals by contacting their MPs via its website.
Meanwhile planning experts have warned the Government's haste to bring in the reforms have worried communities and led to "confusion and uncertainty" for developers.
In a letter to the Daily Telegraph, 23 former presidents of the Royal Town Planning Institute called for calm and offered to bring opposing parties together to resolve the dispute.
The letter states: "The debate about planning reform highlights the importance of planning for growth, building communities and the conservation of cherished spaces. But it has descended into open hostility on all sides, with each party interpreting the policy in different ways.
"The Government wants to bring in a new system almost overnight, reflecting the urgency of delivering localism and stimulating housing and economic growth. While we recognise this urgency and support the Government's overall objectives, the unintended consequences of this haste are greater confusion, uncertainty for the development industry and anxiety for communities."